Some chemicals are very good at killing insects and plants that cause trouble. But the trouble with these chemicals is they can also harm people and the environment. This was the case with the pesticide D.D.T. It is now banned in some countries. But others still use it. Another example involves the widely used pesticide methyl bromide.

In nineteen-eighty-seven, nations met to discuss evidence of a decrease in the level of ozone in the atmosphere. Ozone helps protect against skin cancer from the sun. Negotiators met in Montreal. They developed the first part of an agreement called the Montreal Protocol on Substances that Deplete the Ozone Layer. This treaty sets out steps to protect the ozone in the atmosphere.

In nineteen-ninety-two, methyl bromide joined the list of chemicals to ban. Developed countries agreed to end the use of methyl bromide by two-thousand-five. Developing nations have ten more years.

The Bush administration says it will seek permission for some uses of methyl bromide at least until two-thousand-six. The administration says no replacement can be found for some farm uses. Other nations are also expected to seek special permission.

One-hundred-eighty-three nations have signed the treaty. The United States Congress amended federal pollution laws in nineteen-ninety-five. It did so to meet the requirements of the Montreal Protocol. Since then, the Department of Agriculture has supported programs to find a pesticide to replace methyl bromide.

But its Agricultural Research Service says there is no one chemical or method that can do the job. One of the main problems is that methyl bromide is used on more than one-hundred crops and products. It goes on as a gas in a process called fumigation. The gas works quickly. It is able to kill worms, insects, harmful plants -- many different kinds of organisms. But this poison also affects the nervous system in people and is carried into the atmosphere.

The Montreal Protocol permits countries to continue to use some banned chemicals if they must. Nations that signed the agreement have organized a meeting this month in Montreal to discuss the methyl bromide issue.

This VOA Special English Agriculture Report was written by Mario Ritter. This is Steve Ember.